Each of most optical scanners used in laser beam printers and the like includes a semiconductor laser working as a light source, a rotating polygon mirror, a first imaging optical system for making a light beam emitted from the light source linearly focused on the polygon mirror for correcting a face tangle error of the polygon mirror, and a second imaging optical system for introducing the light beam from the polygon mirror onto a scan surface and focusing the light beam in a uniform spot on the scan surface at an even speed.
Most of the conventional optical scanners are of an under-field type in which the effective opening along a main scanning direction is disposed before the polygon mirror so as to form, on a deflecting/reflecting surface of the polygon mirror, a linear image with a width smaller than the width along the main scanning direction of the reflecting surface. In an optical scanner of the under-field type, however, it is necessary to increase the inner diameter of the polygon mirror when the number of surfaces of the polygon mirror is desired to be increased for increasing the scanning speed. When the inner diameter of the polygon mirror is increased, however, the size of the whole polygon mirror is increased and a large force is necessary for rotationally driving the polygon mirror. Therefore, there is a given limit in increasing the inner diameter of the polygon mirror. Accordingly, the optical scanner of the under-field type has a problem that it is difficult to increase the scanning speed. As a countermeasure, an optical scanner of an over-field type in which a linear image with a width larger than the width of a reflecting surface of a polygon mirror is formed and the reflecting surface of the polygon mirror is regarded as the effective opening along the main scanning direction has recently been proposed (for example, as described in Japanese Laid-Open Patent Publication Nos. 9-304720 and 2000-19443).
An optical scanner disclosed in Japanese Laid-Open Patent Publication No. 9-304720, however, uses an expensive glass fθ lens as a second imaging optical system, and therefore, it has a problem that it is difficult to lower the cost. Furthermore, since an optical scanner disclosed in Japanese Laid-Open Patent Publication NO. 2000-19443 uses a plastic lens, it has a problem that its performance is largely degraded through temperature change.